Sulfite cooking of wood



Feb. 2, 1965 J. J. FERRIGAN, JR., ETAL SULFITE LIQUOR Storage Zone Li uor for sin le Total 50 0f 7.5 st ge acid sulf lte Combined 80 01 1.0% ceasing-(may be use in instant digesting zone or %off f Acid another digesting Dolomltlc |me Sultite zone) or Ammonium KOH ALKALI NOOH SOLUTION NH OH BLENDING 8i SETTUNG At least 1 part by weight of ALKALI of 2% to 10% (PH 6 To 7) solution per 1 part by weight e.g. of SULFITE LIQUOR.

L3 ports SULFITE +2 ports ALKALI SOLUTION 1.?

DIGESTING 2nd Stage ZONE parts WATER. w

WOOD CHIPS-190d SULFITE LIQUOR 3 parts by weight Off 3 parts by weight of spent liquor in digester removed after first stage SPENT LIQUOR added after lst stage (pH below 5) IN VEN TORS iR-kum ATTORNEY United States Patent 3,168,433 SULFITE COOKING 0F WOOD James J. Ferrigan, Jr., Chester, DeL, and Sydney Coppick, Ridley Park, Pa., assignors to Scott Paper Company, Philadelphia, Pa., a corporation of Pennsylvania Filed Dec. 28, 1962, Ser. No. 247,879 3 Claims. (Cl. 162-86) This application is a continuation-in-part of Serial No. 82,388, filed January 13, 1961, now abandoned.

The present invention relates to the manufacture of wood pulp and more particularly to improvements in the sulfite liquor preparation process for cooking of wood whereby a pulp having optimum physical and chemical properties, as well as uniformity of quality and increased yield, results.

Sulfite pulp mills employ a cooking liquor composed of a bisulfite liquor base, such as calcium, calcium-magnesium, magnesium, or ammonia, in a sulfurous acid solution, to delignify wood, usually in the form of chips, under pressure and at an elevated temperature. This cooking operation softens the individual fibers in each wood chip and with solubilization of the bonding lignins, the fibers are easily separated from one another.

It is generally considered that the acid constituent of the cooking liquor penetrates the wood chip more rapidly than the basic constituent and for this reason the first portion of a conventional sulfite cook is conducted at gradually increasing temperatures to a maximum of 110 C. to prevent undue acid hydrolysis and a so-called burned or black cook. Following the penetration period, the digester temperature can be elevated rapidly to about 140 C. and the gauge pressure advanced to about 90 pounds per square inch, such conditions being maintained for the time necessary to complete the delignification process. Usually it is not economically feasible to reduce the lignin content of the wood below about 5%, for the prolonged cooking incidental to such operation will result in cellulosic deterioration and a corresponding loss of yield.

Sulfite cooking is subject to numerous process variables but of major significance is the limited number of wood species adapted to sulfite pulping. The more resinous woods of the Pinus type, as well as Douglas fir and the western hemlocks, do not lend themselves readily to the usual sulfite pulping methods.

It is, therefore, a primary object of the present invention to provide a modified acid bisulfite cooking process adaptable to and permitting a wider utilization of wood species than heretofore could be realized through sulfite pulping.

A further object of our invention is the provision of an improved acid bisulfite liquor for a cooking process which enables better wood chip penetration with a cor responding better yield and pulp of more uniform quality.

A still further object of our invention is the provision of a modified acid bisulfite cooking process which may be employed in conventional and existing equipment.

Other objects and advantages of the invention will be readily apparent from the following detailed description of certain preferred embodiments thereof.

Briefly stated, the present invention contemplates .a liquor for a two-stage digestion in which initial and substantially complete wood penetration with the basic constituents of a modified sulfite cooking liquor is followed by acidic chemical reactions incident to sulfite pulping.

Our pulping process is suitable for hard woods, including oaks, maple, beech, aspen and willow, as well as the soft woods, such as, spruce, fir, hemlock and pine. Usually the wood is in the form of chips, classified as to size (preferably under A" in length) to promote uniformity of penetration by the digesting liquors.

Patented Feb. 2, 1965 In a pulping process adaptable for pulping wood in two-stage pulping process whereby better chip penetration, better useable yields, and pulp of more uniform quality results, the improvement comprises the following steps: Sulfite liquor which is suitable for one-stage pulping process is used. This acid sulfite-containing liquor is selected from at least one member of the group consisting of calcium, dolomitic limestone, and ammonium acid sulfite liquors. As practiced herein the acid sulfite solution contains about 1% combined sulfur dioxide, about 6.5% free sulfur dioxide and about 7.5% total sulfur dioxide. The liquor is separated into two streams. The first stream is modified by adding to one part by weight of it in excess of one part by weight an alkali solution of an alkali hydroxide selected from at least one member of the group consisting of potassium hydroxide, sodium hydroxide and ammonium hydroxide in an amount such that the pH of the modified liquor is from about 6 to about 7. In general the concentration of the alkali in this solution is of from 2% to 10%. Thereafter the modified liquor is settled and used in a pulpyielding material. After cooking, a part of the modified liquor is removed and the second stream of said onestage liquor is added to the remaining solution in the cooking zone in an amount such that the pH of the solution is below 5. Pulping for a time sulficient to yield an increased amount of pulp is then continued. The present process may be run by alternating in the same digesting zone the acid sulfite pulping with the novel process of pulping, Similarly, the acid sulfite pulping and the novel process may be carried out in parallel in an existing mill having two digesting zones. This flexibility in mill operation isnow possible by using the modified liquor and the conventional acid sulfite liquor.

In the accompanying drawing forming a part of this disclosure, the figure is a diagrammatic representation of the invention involving the present process depicted as a flow sheet.

Starting acid sulfite liquor is obtained from sulfite liquor storage zone. A portion of this liquor is drawn off if needed for a single-stage acid sulfite cooking. A portion of the remaining liquor is blended with an alkali solution and thereafter settled. This primary liquor is used in the first-stage digesting of pulp-yielding chips. A portion of this liquor is withdrawn after the first-stage digesting and this liquor may be reused. The liquor for the second stage is obtained by adding to the remain-' ing liquor in the digesting zone the acid sulfite liquor from the sulfite liquor storage zone in an amount such that the pH is below 5. The spent liquor and pulp are thendischarged from the digesting zone.

. In the preferred embodiment of our invention, the

cooking liquor, an alkaline earth metal'bisulfite, in an amount equivalent to about 1.3 times the weight of the wood chips (O.D.) to be digested, is combined with an alkaline hydroxide solution, such as, potassium hydroxide, sodium hydroxide, or ammonium hydroxide, in an amount equivalent to from 2% to 10% of the weight of the Wood chips, and in a concentration such that the combined liquors have a pH in the range of from 6.0 to 7.0.

The concentration of the alkaline hydroxide solution should also be selected so that on mixing with the bisulfite cooking liquor a total quantity of liquor is obtained which is equivalent to 5 times the weight of wood chips (O.D.) to be digested.

The bisulfite cooking liquor and the supplemental quantities of alkaline hydroxide may be added separately to the digester after it has been loaded with wood chips, or the two components of the cooking liquor may be preblended in a separate mixing tank and subsequently charged into the digester with or without separathe equilibrium steam pressure is generally adequate for liquor diffusion throughout the wood chips, such action will" be expeditedun'der a pump-induced pressure of from 90 to 100 pounds per square inch gauge.

At theend ofthe primary penetration stage, the pressure loading of the digester is relieved and any excess cooking liquor removed. This'cooking liquor is either discarded or returned to a holding tank from which it may be drawn to make up new quantities of cooking liquors for subsequent operations.

Digestion of. the impregnated wood chips is completed by charging the digesterwith conventional acid sulfite liquor in sulficient quantity that the final liquor to wood ratio is about 5 to 1. The digester temperature is again raised to from 130 C. to 150 C. and subjected to a pressure of between 90 and 100 pounds per square inch gauge for a further period of from 1 .to 3 hours. Thereafter, the temperature and pressure are reduced and the contents of the digester are blown or dumped following the normal procedures which are well known to those skilled in the art. During this secondary cooking stage the wood chips are penetrated ,by'the acid components of the cooking liquor- Even and uniform cooking results, free from the likelihood of differential penetration rates so 'difiicult to control in conventional sulfite pulping.

The following examples are illustrative of the various facets of our invention.

Example I A dolomitic limestone with a chemical composition of CaO and MgO in the ratio 'of3 to2 was employed to prepare .a cooking liquor. having a combined S content of 1% and a. total S0 content of 7.5%. To 1.3 parts by weight of thiscooking liquor; contained in a h'olding'tank, were added 2 parts by weight of a 5% caustic soda solution and 1.7 parts by weight'of water; The composite liquor was settled and the clear solution tra'nsportedto a digester which was charged with 1 part byweight equivalent of dry wood chips prepared from northeastern bark stained spruce woodaveraging 100 years in age. The digesterwas heated via indirect steam and forced circulation so that the temperature was raised to 130 C. in 1.5 hours at which point. it was held for 4hours'with 'an over-pressure maintained at 95#/in.

for 2% hours. The washed and screened pulp gave the following results:

Yield of screened pulp "percent" 56.3 Yield of screenings do 0.24 Permanganate number 19.8

Example III Ammonium hydroxide was employed rather thanthe dolomitic limestone of Examples I and II to prepare a cooking liquor having a chemical composition of 1% combined and 7.5% total S0 The primary cooking liquor was combined with 5% causticsoda'solution'and water, as

' in Example I, and the "mixturewas added directly to a digester containing the bark stained sprue wood chips without pre-settling. The primary cooking stage was conducted at 130 C. for 4'hours and the secondary cooking stage at 145 C. for 3 hours as in -Example I.' The washed and screened pulp gave the following results:

Yield of screened pulp a pe'rce'nt 56.4

Yield of screenings ..do 0.50

Permanganate number 21.4

' Example IV Bark stained spruce wood was pulped as in'Exarnple 'IlL'with the exception that the secondary cooking stage was carried out for 3 /2 hours at 145 C. The resulting screened pulp had the following properties:

Yield of screened pulp percent.. 53.4

Yield of screenings I do 0.16

Permanganate number 16.3

Example V An ammonia'base starting liquorwas prepared as in 7 Example The composite primary cooking liquor was prepared from 1.3 parts by weight of cooking liquor, 2.4

parts by weight of 10% aqueous ammonia and 1.1 parts by weight of water; This solution was added directly without pre-settling to a digester containing 1 part by* weight on a dry basis of bark stained spruce wood chips.

At the end of this primary coking stage, 3 parts by weight g of the liquor were. drained from the digester. Then 3 7 parts by weight ofun'rnodified cooking liquor were injected into the digester and cooking resumed with indirect steam and forced circulation. The temperature was raised rapidly to 145 C. andmaintained for 3 hours with the pressure regulated at 90#/in'. The pressure andjtemperature were then reduced and the contents of the digester dumped. The pulp wa'slwashed and screened and upon" analysis gave the following results:

Yieldof screened pulp percent-.. 53.7

Yieldof screenings do 0.26

Permanganate number l 15.5

Example ll Bark stained spruce wood was digested as in'Example I with the exception that the primary digestion was carried out for 4 /2 hours at a maximum temperature of 130 C., while the'seconda'ry digestion was conducted at 145C.

A starting liquor from dolomitic limestone was prepared asin Example I. Bark stained spruce wood'chips were digested with this'liquor in a single stage cooking cycle typical of present day practice, wherein'the'tem- Yield olf screenings do 1.9" Permanganate number- 11.7

Example VII 'Ifiianother digestion of bark stained spruce wood witha starting liquor fromdolomitic limestone as in Example VI, th'e'cooking schedule again'followed a typical present day practice with 'a' temperature" rise to 136 C. in 4%* hours at whichpoin't it was held for 1 /2 hours. After.

dumping, washing and screening, the following results were obtained:

Yield of screened pulp percent 45.3 Yield of screenings do 3.4 Permanganate number 15.3

Example VIII Northeastern poplar wood chips were cooked in the first stage with 1.3 parts by weight of a dolomitic limestone base acid sulfite cooking liquor, as in Example I, to which was added 2.4 parts by weight of aqua ammonia and 1.1 parts by weight of water for each part by Weight of wood calculated on the dry basis as in Example V. Digestion was carried out with this settled liquor for 4 hours at 130 C. and after drawing down as in Example V to a liquor to wood ratio of 2 to 1, 2% parts by weight of unmodified cooking liquor were injected into the digester and cooking contintued for 3 hours at 145 C. The pulp obtained gave the following results:

Yield of screened pulp percent 53.0

Yield of screenings do 0.06

Permanganate number 7.1

Example IX Northeastern poplar wood was cooked in the first stage with 1.3 parts by weight of dolomitic limestone base acid sulfite cooking liquor to which had been added 2 parts by weight of 5% caustic solution and 1.7 parts by weight of water and settled as in Example I. Cooking was confollowing results:

Yield of screened pulp. percent 54.5

Yield of screenings do.. 0.02

Permanganate number 7.9

Example X Poplar wood was cooked via the two stage process, as in Example IX, with the exception that the second stage was conducted for 2 hours at 145 C. The following results were obtained:

Yield of screened pulp percent 56.1

Yield of screenings do 0.08

Permanganate number 10.8

Example XI Poplar wood was cooked as in Example VIII, with the exception that the second stage was conducted at 145 C. for 2 hours, the following results were obtained:

Yield of screened pulp percent 55.2 Yield of screenings do 1.0 Permanganate number 12.9

Example XII Poplar wood was cooked with an ammonia base acid sulfite liquor to which aqua ammonia had been added as in Example V. The second stage was conducted as in Example V with the exception that the digester was maintained for 3 hours at 145 C., with a liquor ratio of 4.5

to 1. The following results were obtained:

Yield of screened pulp percent 55.7 Yield of screenings do 004 Permanganate number 9.98

Example XIII Poplar'wood was cooked with an ammonia base acid sulfite starting liquor to which caustic had been added as in Example HI. The second stage was conducted at Example XIV Poplar wood was digested with a dolomitic limestone base acid sulfite liquor according to normal present day practice, as in Example VI, with the exception that the top temperature was 134 C., and this was maintained for 1 /2 hours. The following results were obtained:

Yield of screened pulp percent 49.6 Yield of screenings do 0.16

Permanganate number 5.2

Example XV Poplar wood was cooked as in Example XIV with the exception that the maximum temperature was maintained for 1 hour. The following results were obtained:

Yield of screened pulp percent 51.4 Yield of screenings do 0.70 Permanganate number 8.22

Example XVI Jack pine chips were pulped as in Example I with the exception that the primary partial digestion was conducted for 2% hours with maximum temperature of 146 C., while the secondary digestion was performed for 3 hours at 146 C. The following results were obtained:

Yield of screened pulp percent 49.0 Yield of screenings do 0.8 Permanganate number 14.8

Example XVII Douglas fir chips were pulped as in Example'XVI. The following results were obtained:

Yield of screened pulp percent 53.5 Yield of screenings do 0.1 Permanganate number 21.7

Example XVIII Western hemlock chips were pulped as in Example XVI. The following results were obtained:

Yield of screened pulp percent 45.7 Yield of screenings do 0.1 Permanganate number 15.1

Example XIX Loblolly pine chips were pulped as in Example XVI. The following results were obtained:

Yield of screened pulp percent 48.0 Yield of screenings do 0.6 Permanganate number 21.6

The foregoing examples clearly demonstrate the advantages of our two stage modified liquor pulping process and further serve to distinguish it from the conventional sulfite cooking methods. An improved yield of usable pulp is obtained with those wood species which are normally susceptible of acid bisulfite cooking, while other more diflicult to pulp woods also react satisfactorily and in adequate yield. The low permanganate number of the several pulps establishes their ready bleachability with the usual peroxide hypochlorite and chlorite agents.

The final pulps, either bleached or unbleached, can be made into papers which possess physical characteristics comparable to those produced from ordinary acid sulfite cook pulps and thus it is established that our modified process has no deleterious effects upon the wood constituents.

It will be obvious that various modifications to the hereinbefore described pulping: procedures and the composition of the cooking liquors per se are possible without departing from the spirit of our inventionv or the ing liquor to produce alkali-modified-liquor having a pH of from about 6 to about 7;

(b) cooking pulp-yielding material in the presence, of the alkali-modified liquorto efiect penetration of the.

pulp-yielding material therewith;

(c) withdrawing excess liquor from the cooked pulpyielding material;

(d) adding acid sulfite cooking liquor to the penetrated and cooked "pulp-yielding material in sutficient amount to adjust the pHof the resultant mixture to a value below 5; t

(e) vcontinuing the pulping. process while maintaining the pH of the aforementioned resultant mixture be-- low '5.

2. A process of preparing and utilizing liquor for cooking pulp-yielding material, comprising the steps of:

, (a) separating-an acid-sulfite cooking liquor into first and second streams, said liquor being selected from atleast one member of the group consisting of calcium, dolomitic limestone, magnesium and ammonium acid sulfite liquors;

(b) combining said first stream of cooking liquor with sutficient alkali hydroxide solution to produce alkalimodified liquor having a pH of from about 6 to about 7; V

(c) settling the alkali-modified liquor;

(d) subjecting V pulp-yielding material to, a primary cooking stage inthe presencerof the settled alkalimodified liquor to eifect penetration of the pulpyielding material therewith;

(e) withdrawing excess liquor from the penetrated pulp-yielding material;

(f) adding the vsecondwstream of acidysulfite cooking liquor to the penetrated. pulp-yielding material in sufiicient amount to adjust the pH of the resultant mixture to a value below 5; and

(g) continuing-the pulping process in a secondary cooking stage while maintaining the pH of the aforementioned resultant mixture below 5.

and utilizing liquor for cook 3. A process of preparing and utilizing liquor for cook-1;

ing pulp-yielding material, comprising the steps of:

- (a) combining acid sulfite cookingliquor selected from at least one member of the group consisting of cal cium, dolomitic limestone, magnesium and ammonium acid'sulfitebooking liquors and containing about 1% combined sulfur dioxide, about 6.5% free sulfur dioxide and about 7.5% total sulfur dioxide,

with sufiicient alkali hydroxide solution selected fromat-least one member-of the group consisting of potassium: hydroxide, sodium hydroxide, and ammo; nium hydroxide] solutions having a weight concen-v tration of 2% to about 10%, said hydroxide solu-,

tion being employed in sufficient amount to produce an alkali-modified acid sulfite. cooking liquor having;

7 a pH of from about 6 to about 7;-

(b) settling the alkali-modified liquor;

(0) adding the settled alkali-modifiedliquorto pulpyielding material in a digestion zone; (d) subjecting the pulp-yielding material to a primary cooking stage'in the digestion zone in the presence of the settled alkali-modified liquor toetfect pene-- trationof the pulp-yielding material therewith;-

(e) withdrawing excess liquor from the penetrated pulp-yielding material;

(f) adding to the penetrated pulp-yielding material in said digestion zone in an amount suflicient to produce in the resultant mixture a pH of less than 5, acid sulfite cooking liquor selected from at least one member of thegroup consisting of calcium, dolomitic limestone, magnesium and ammonium acid sulfite cooking liquors and containing about 1% com-' bined sulfur dioxide, about 6.5% free sulfur dioxide and about 7.5 total sulfur dioxide;

(g) continuing the pulping process in a secondary cooking stage while maintaining the pH of the aforementioned resultant mixture below 5.

References Cited by the Examiner UNITED STATES PATENTS 1,848,661 3/32, Richter. 162-.,-84 1,922,160 1 8/33 .7 Haglund 162-r84r 2,885,317 5/59 Collin .etal 162-184,;

FOREIGN PATENTS 552,161 1/58 Canada.

DONALL H. SYLVESTER, Primary Examiner. MORRIS o. WOLK, Examiner. 

1. A PROCESS OF PREPARING AND UTILIZING LIQUOR FOR COOKING PULP-YIELDING MATERIAL, COMPRISING THE STEPS OF: (A) COMBINING SUFFICIENT ALKALI WITH ACID SULFITE COOKING LIQUOR TO PRODUCE ALKALI-MODIFIED LIQUOR HAVING A PH OF FROM ABOUT 6 TO ABOUT 7; (B) COOKING PULP-YIELDING MATERIAL IN THE PRESENCE OF THE ALKALI-MODIFIED LIQUOR TO EFFECT PENETRATION OF THE PULP-YIELDING MATERIAL THEREWITH; (C) WITHDRAWING EXCESS LIQUOR FROM THE COOKED PULPYIELDING MATERIAL; (D) ADDING ACID SULFITE COKING LIQUOR TO THE PENETRATED AND COOKED PULP-YIELDING MATERIAL IN SUFFICIENT AMOUNT TO ADJUST THE PH OF THE RESULTANT MIXTURE TO A VALUE BELOW 5; (E) CONTINUING THE PULPING PROCESS WHILE MAINTAINING THE PH OF THE AFOREMENTIONED RESULTANT MIXTURE BELOW
 5. 